Process control systems, like those used in chemical and petroleum processes, typically include one or more process controllers communicatively coupled to at least one host or operator workstation and to one or more field devices via analog, digital, or combined analog/digital buses. The field devices, which may be, for example, valves, valve positioners, switches, and transmitters (e.g., temperature, pressure, and flow rate sensors), perform functions within the process plant, such as opening or closing valves and measuring process parameters. The process controllers receive signals indicative of process measurements made by the field devices and/or other information pertaining to the field devices, use this information to implement control routines, and subsequently generate control signals that are sent over the buses or other communication lines to control the operation of the field devices. With the information collected from the field devices and process controllers, an operator or a technician can execute one or more applications at an operator workstation that perform any desired function with respect to the process, such as, for example, configuring the process, viewing the current state of the process, and/or modifying the operation of the process.
In many cases, field devices may require on-site setup, configuration, testing, and maintenance. For example, before a field device can be installed at a particular location at a process control plant, the field device may need to be programmed and may then need to be tested before and after the field device is installed. Field devices that are already installed may also need to be regularly checked for maintenance reasons or, for example, when a fault is detected and the field device needs to be diagnosed for service or repair. Generally speaking, configuration and testing of field devices are performed on location using a handheld, portable maintenance tool. Because many field devices are installed in remote, hard-to-reach locations, it is more convenient for a user to test the installed devices in such remote locations using a handheld, portable tool rather than using a full configuration and testing device, which can be heavy, bulky, and non-portable, generally requiring the installed field device to be transported to the site of the diagnostic device.
In the case in which a field device is at least partially operational and supplied with power via a local bus, a handheld maintenance tool or portable testing device (“PTD”) can connect to a communication terminal of the field device to run a diagnostic routine. Generally, the field device and the PTD communicate over a two-wire or a four-wire communication connection or line, typically referred to as a bus. For example, FOUNDATION® Fieldbus devices and HART® devices are typically connected to a two-wire (or in some cases a four-wire) connection line or bus when installed in a plant environment. It is known to use a handheld device to connect to, for example, a Foundation Fieldbus or a HART communication line or other communication bus to communicate with devices connected to that communication line or bus.
In some cases, Intrinsic Safety (“IS”) standards restrict the manner in which power and other communication signals can be provided to a field device, especially when the field device is installed in a critical or dangerous process control system in the field. Generally, higher voltages are used for providing power to the field devices than the voltages that are used for communicating with the field devices. Additionally, certain safety measures must be implemented before powering a field device in the field. In particular, according to IS guidelines, a technician cannot switch on the power of a field device within the field device itself and cannot use devices that generate voltages over certain predetermined levels. The IS guidelines prohibit internal power switching and generation of larger voltages because field devices are often installed in proximity to volatile substances or volatile processes, and thus there is higher possibility of causing an explosion by arcing or generating sparks when a high voltage or a power connection is applied to the field device. For reference, an internal switch may be considered any switch that is integrally connected within or physically housed within a field device and/or that is fixed to the field device.
Related IS guidelines also advise against switching on power within a PTD that is connected to a field device and that is located within a vicinity of the field device. IS standards generally require manual intervention when applying power to a non-operating or a non-powered field device installed in the field. Although it may be desirable to configure existing PTDs with automatic power functions for powering a field device, this configuration is generally prohibited under the IS standards, especially when providing higher power signals to the field devices for powering the field devices or for testing purposes.
To comply with IS standards, some existing PTDs include an interface with four connection ports for coupling four lines or wires between the PTD and a field device undergoing testing. Generally, a first pair of lines is used for transmitting communication signals at a first voltage range and a second pair of lines is used for powering the field device at a second and higher voltage or voltage range. The first pair of lines is primarily used whenever the field device is undergoing testing, and the second pair of lines/wires is used only when power is needed to be provided to the field device to enable the field device to execute a function (e.g., a test function or a configuration function). In this manner, additional power to the field device undergoing testing always requires manual intervention that includes connecting additional wires between the field device and the PTD. In short, IS standards have generally limited the development of portable field device testing equipment to require two separate sets of lines or lead sets and three or four ports for connecting a field device to the portable testing equipment.
In any event, it is difficult, if not impossible, to use handheld devices to communicate with field devices via an installed communication line or bus if the communication line or bus has a fault, such as a short circuit (a low impedance) fault or an open circuit (a high impedance) fault. Moreover, when such a fault exists in a communication bus, it can be difficult to detect the existence of the fault in the bus or the communication line in the first case. For example, a handheld device may be able to connect to and operate on the bus, and may even be able to communicate with some of the devices on the bus, when the bus experiences a high impedance fault. In these cases, it is difficult for the operator to know whether the inability to communicate with a field device on the bus is being caused by a fault in the bus or by a fault within the device on the bus. Still further, even if the operator knows that there is a fault in the communication line or bus, it is difficult for the operator to know where the fault exists and thus be able to easily find and repair the fault. In some cases, the communication and power lines of process control communication busses can traverse great distances within a plant, and these lines may be hidden, covered or otherwise hard to see, as installed in the plant. Thus, it may take a long time to visually inspect a line to find a fault, even if the operator knows a fault exists within the line.